Manufacture of polyamide structures



Patented Feb. 1, 1944 UNITED STATES PATENT OFFICE MANTJFACTURE orrornmnn s'raucroans Orrlngton Embry Dwyer, Rochester, N. Y., as-

signor to E. I. du Pont de Nemours & Company, Wilmington, Del., acorporation of Delaware No Drawing. Application April 25, 1942, SerialNo. 440,482

18 Claims.

'will be discussed with particularreference to the production oftransparent synthetic linear polyamide sheets and films. It is to beunderstood,

however, that the invention isnot so limited.

The production of transparent synthetic linear polyamide sheets andfilms has been known prior to the present invention; however, suchpreviously known processes have certain disadvantages. For example, theproduction of transparent synthetic linear polyamide films by a processof melt casting is disclosed in Foster US. Patent No. 2,212,770. Themelt casting of films of any substantial width, however, presentsinnumerable problems. Difiiculties have consequently been. encounteredin the commercial exploitation of such a process.

The copending application of Orion E. Schupp, Jr., Serial No. 362,660,filed October 24, 1940, discloses a method of dissolving syntheticlinear polyamides in solutions of metal salts in alcohols. Suchsolutions can be readily used for the extrusion and casting of films andsheets. However, by the casting of sheets and films in accordance withthe above-identified Schupp process, extruded or cast sheets and filmswill be opaque in nature; For certain purposes, it is highly desirablethat films and sheets composed of synthetic linear polyamides be clearand transparent.

It is therefore an object of the present invention to provide a methodof producing transparent articles, particularly sheets and films, frommetal salt-alcohol solutions of synthetic linear polyamides.

Other objects of the invention will appear hereinafter.

By the-term, synthetic linear polyamides, as used throughout thespecification and claims, is meant synthetic linear polyamides of thetypes described in U. S. Patents, Nos. 2,071,250, 2,071,- 253 and2,130,948. These polyamides contain amide groups o R l where R ishydrogen or a monovalent hydrocarbon radical) as an integral part of themain chain of atoms in the polymer. Further, the av erage number ofcarbon atoms in thesegments of the chain separating the amide groups inthese polyamides is at least two.

In accordance with the present invention,

transparent articles can be produced from metal salt-alcohol solutionsof a synthetic linear poly-' amide by dissolving'a synthetic linearpolyamlde havinga quench index above 125 mils in a solution taken fromthe .group consistingof metal salt-methanol and metal salt-ethanolsolutions. forming an article from said solution, evaporatingsubstantially, all of the alcohol from the formed article at a ratesufilciently slow to prevent the formation of bubbles therein and beforethe article comes into contact with'water for an atmosphere having arelative humidity above 50%, then washin g the metal salt from theartiole with water having a temperature not to iexl5 ceed 40 C. andfinally drying .the article.

The iollowing specific examples illustrate oer 1 tain preferred methodsfor carrying out the present invention, it being understood thattheinvention is not to be limited to the: details Set forth in theseexamples.

Example I anol. The calcium chloride used in the solution :0 was firstfused to remove all water therefrom. and the methanol used in thesolution'was-first dehydrated by distilling the same over magnesiummetal. A glass plate is coated with a quantity oi the solutionand'excess solution is re moved by means of a doctor knife. The coatingis carried out while the glass is heatedto a tern perature ofapproximately 50 C. and the sur= rounding atmosphere is similarly heatedwith air having arelative humldityof 5%. The'tempjer,

4o ature oi the glass plate and the surroundingfatmosphere ismaintainedgat' 50 C. forra period ofabout 10 minutes and the temperature.of'the plate-and the surrounding atmosphere is then raised to 100 C.The plate and coating is heated at 100 C. for sufli'cient time for-theremoval of substantially all of the methanol; The above method ofexposing the plate and coating. to' heated air will avoid too rapidevaporation oi the-methanol and thereby prevent; formation of bubblesinthe film. Thefilm is glass clear and is washed in water having atemperature of18 j 0. until substantially all, of the calcium chloridehas beenremoved. The film isthen removed from. the glass plate. Thestill glass clearfllm is dried under a slight tension (sufllcienttension to prevent shrinkage and wrinkling? of the film). The resultingfilm is glass clear and has very satisfactory physical properties.

Example I! A polyamide-metal salt-methanol solution is preparedsimilarly to the method outlin d in Example 1' except that the methanolis e commercial grade of synthetic methanol containing approximately0.5% water. The glass clear film is prepared dn the same manner as setforth in Example I except; that the calcium chloride is removed from thefilm by washing the same in water having a temperature of C. Theresulting film has the same glass clear transparquenched by dropping itinto a mass of water having a temperature of 0 C. The aluminum is -oropaque because of crystallization The line of demarcation between clearand cloudy polyamide ency and good physical characteristics of the filmobtained by the process of Example I.

Example III Five hundred parts of an interpol'ymer prepared fromhexamethylene diammonium adipate, hexamethylene diammonium sebacate, and

caprolactam, the proportions of these ingredients in the ratio of 40parts to 30 parts to 30 parts, respectively, are dissolved in a solventcomprising 2500 parts of calcium chloride and methanol. The calciumchloride constituted of the methanol-calcium chloride solution. Thepolyamide solution is coated on-a glass plate and processed in themanner set forth in Example I. A glass clear film having desirablephysical characteristics is thereby obtained.

For the production of transparent synthetic linear polyamide articles inaccordance with the present invention, it is preferred to use aninterpolyamide prepared from hexamethylene diammonium adipate andhexamethylene diammonium sebacate, preferably in the proportion of 30%of the former and 70% of the latter. It is nevertheless possible to useinterpolymers containing the above two ingredients in a range between15% of the former and 85% of the latter to of the former and of thelatter. The above range represents the most readily meltableinterpolymers which can be prepared from the above two compounds.

Although the above-named interpolyers are particularly useful for thepurpose of the present invention, it is within the scope of theinvention to use any synthetic linear polyamide including syntheticlinear interpolyamides provided that the polyamide has a quench indexabove 125 mils, and provided that it is film-forming or fiber-forming incharacter and that it is soluble in a metal salt-alcohol system of thetype disclosed in the above-identified Schupp application. Whether. asynthetic linear polyamide has a quench index of above 125 mils can bedetermined in the following manner:

Sheet aluminum of 0.008 inch thickness is wrapped around a steel wedgehaving a width of approximately 1% inches, a length-of approximately 8inches and an included angle of 2.5. The seams of the aluminum sheet arelocked and hammered so that awater-tight mold is constructed. This moldis filled-with flakes of the synthetic linear polyamide to be tested,and the mold is placed in a test tube which is evacuated and heated bypositioning the --same within a bath of diphenylene oxide vapor forminutes to melt the polyamide. The vacuum line to the test tube is thenclosed and carbon dioxide is slowly admitted to the tube. Twenty secondsafter shutting oil the vacuum, the wedge is can be easily seen andmarked with ink. The thickness of the wedge at various points along thisline is measured, in mils, witha micrometer. An average of four suchmeasurements is usually sufiicient and is taken as the quench, index'ofthe polyamide. The quench index of a given polyamide can be reproducedwithin about 15% of a given value.

Example IV A polyamide-metal salt-methanol solution is preparedsimilarly to the method outlined in Example I; The polyamide solution iscast from a hopper through a narrow slit on to a metal casting belt. Thebelt carrying the polyamide passes into a heated zone where thetemperature of the air surrounding the belt increases progressively from50 C. to C. in the direction of movement of the belt. The speed of thebelt is such that the film, when it reaches the end of the heating zone,will be substantially free from methanol. The film on thebelt is thensubjected to washing by means of a countercurrent flow of water having atemperature of 9 C. to remove the salt from the film. The film on thebelt is then subjected to drying by passing it into a second drying zoneto remove substantially all of the water therefrom. The dried film isthen stripped from the belt and wound on a support in a conventionalmanner. The resulting film has the same glass clear transparency andgood physical characteristics of the film obtained by the processorExample I.

Reference is made in the examples to the substantially complete removalof the methanol before the film is contacted with water or an atmospherehaving a high relative humidity. It is preferred that the film be heateduntil substantially all of the methanol is removed before the filmcontacts water or atmosphere having a relative humidity in excess of50%. The sufilciency of the removal of the methanol to insure productionof a transparent film may be readily determined in an empirical mannerfor a given set of evaporating conditions. For example, test samples maybe taken from the film after various intervals of time exposure, forexample, 1, 2, 5, 10, 20, 30 or 60 minutes after exposure to a certainset of evaporating conditions, and these test samples washed and driedto observe trans-,

parency of the film.

In the above examples, specific reference was made to the use ofsynthetic linear polyamides dissolved in calcium chloride-methanolsolvents. Numerous other metal salt-methanol and metal salt-ethanolsolvents may be used in the same manner. The above-identified Schuppapplication discloses other metal salts which may be used as equivalentsfor calcium chloride. For example, zinc, magnesium, lithium, "strontium,aluminum, titanium, bismuth. cobalt and iron, bromides, iodides,nitrates and thiocyanates may be used. The only alcohols suitable foruse in accordance with the present invention are methass-10,652 I thatequally desirable resultsmay be produced anol and ethanol. Theconcentration of polymer in the solvent may vary between wide limits. Itis only necessary that the solution may be coated to produce a film ormay be spun to form a thread, or formed in the shape of other articles.It is. desirable, however, to use solutions comprising from to 35%, andpreferably from to of the polyamide, and it is also preferred that themetal salt concentration used be that which will give substantially thehighest concentration of polymer in the solution. In preparing thepolyamide solutions, it is preferred that the metal salt and alcohol besubstantially free of water. Thisis not essential however. The polyamidesolution may contain up to 2% and possibly 3% water without preventingthe formation of a transparent film in accordance with the process ofthe present invention.

Although it is preferable because of ease of operation to remove thealcohol as completely as possible prior to the first contactv of thefilm with moisture, nevertheless, film of good clarity can be obtainedeven though traces of the alcohol are still present in the film at thetime of washing. If slight traces of alcohol are present in the film, itis preferred that the film be quenched with water having a temperaturebetween 10 C. and the freezing point of the wash water. Thereafter, itmay be washed with water at temperatures as high as 40 C. If the film isheated to a temperature above about 50 C. prior to the washing step, itis preferred that the film be first cooled to a temperature oi. about 25C. to C. before washing the same in wash water at about 25 C. or lessfor removal of the metal salt.

As stated above, it is highly desirable to evaporate the alcohol fromthe formed article at a rate sufiiciently slow to prevent the formationof bubbles therein. This is particularly true where it is desirable tohave a glass clear transparent article since the formation of bubbles inan article by reason of excessively high rate of evaporation will causethe film to be hazy or even opaque.

It is furthermore desirable to remove substantially all of the alcoholbefore the article comes into contact with water or an atmosphere havinga relative humidity above 50%, in other words. before the article comesinto contact with moisture in greater concentration than an atmos--phere having a relative humidity above 50%. Heretofore, it has beenconsidered desirable to first wash the metal salt from the article whileappreciable quantities of alcohol are still present in the article andthen dry the article and thereby remove the quantlties of alcoholremaining therein. By this procedure, however, it is found thattransparent articles will not be produced. It is, therefore, highlyimportantthat the alcohol be substantially completely removed from thearticle before contacting the same with moisture. By the termsubstantially completely removed" it is meant that no more than slighttraces of alcohol remain in the article.

With regard to the tension used during the drying of film, or similararticles, there is no objection to applying considerable tension to thefilm. It is important, however. that the film be. held sufiiciently tautto prevent the formation of wrinkles therein which would normally becaused by drying a film in a relaxed condition.

For convenience, this invention has been discussed with particularreference to the production of sheets or films. It is to be clearlyunderstood, however, that the invention is not limited to the productionof sheets or films, and

if articles of, other forms are produced in accordance with thepresent'lnvention. For example, the invention is'applicable to theproduction of yarns, fibers andfilms.

As above stated, after the alcohol is substan-,

. acid, and silver nitrate added to the solution.

If no more precipitate is obtained than is ob'- tained in acorresponding test run with a sample of water containing not to exceedparts per million calcium chloride, the calcium chloride has beenadequately removed.

The present invention provides a simple, unique, and practical processfor producing transparent synthetic linear polyamide articles. Theproduction of such transparent polyamide articles has long been desiredand the present invention makes such desires of the public obtainable.The transparent films of synthetic linear polyamides produced inaccordance with the present invention are comparable in strength,elongation, and ultraviolet light transmission to regenerated cellulosefilm of similar thickness. These polyamide films are superior toregenerated cellulosefilms in tear resistance, water impermeability andflexibility. Transparent synthetic linear polyamide films produced inaccordance with the present invention can be used' may be formed intocups or the like articles by the use of a. drawing punch and die such asthe type designed for use in the die stamping of aluminum. These filmsmay also be used in the production of transparent tubing by bringing theedges of the polyamide sheeting together continuously and passing theseam first between hot rolls and then between cold rolls, or over amandrel, first under a hot roll and then under a cold roll. Such tubingis capable of being oriented by drawing and it is possible to preventcollapsing of the walls-of the tubing during the drawing operation byfilling the bore with a liquid, such as water, during this operation.Another use to which these films can be put is in the production ofartificial lace and the like articles wherein 'a design is cut inthefilm by means of a blade or plurality of blades. The cutter may be inthe form of a rotary cutteror a stamping die. If desired, the cutter maybe heated to produce fusion of 'the edges of ,the polyamide film as itis cut.

Since it is obvious that many changes and modifications can be made inthe above-described details without departing from the nature and spiritof the invention, it is to be understood that the invention is not to belimited to the details described herein except as set forth in the appended claims.

I claim:

1. The method of producing atransparent arquench index above 125 mils ina solution taken from the group consisting of metal salt-methanol andmetal salt-ethanol solutions, forming an article from said solution,evaporating substantially all the alcohol from said formed article at arate sufiiciently slowto prevent the formation of bubbles therein andbefore said article comes into contact with moisture ingreaterconcentration than anatmosphere having a relative humidity above50%, and washing the metal salt -from said article with water having'atemperature not to exceed 40 C., and drying said article.

2. The method of producing a transparent article from a metalsalt-alcohol solution of a synthetic linear polyamide which comprisesdissolving a synthetic linear polyamide having a quench index above 125mils in a calcium chloride-methanol solution, forming an article fromticle with water having a temperature not to exceed 40 0., and dryingsaid article.

3. The method of producing a transparent article from a metalsalt-alcohol solution of a synthetic linear polyamide which comprisesdissolving a synthetic linear interpolyamide having a quench index above125 mils in a calcium chloride-methanol solution, forming an articlefrom said solution, evaporating substantially all the alcohol from saidformed article at a rate sufficiently slow to prevent the formation ofbubbles therein and before said article comes into contact with moisturein greater concentration than an atmosphere having a relative humidityabove 50%, an washing the metal salt from said article with water havinga temperature not to exceed 40 C., and drying saidarticle.

4. The method of producing a transparent article from a metalsalt-alcohol solution of a synthetic linear polyamide which comprisesdissolving an interpolymer prepared from hexamethylene diammoniumadipate and hexamethylen diammonium sebacate having aquench index above125 mils in a calcium chloride-methanol solution forming an articl fromsaid solution, evaporating substantially all the alcohol from saidformed article at a rate sufilciently slow to prevent the formation ofbubbles therein and before said article comes into contact with moisturein greater concentration than an atmosphere having a relative humidityabove 50%; and washing the metal salt from said article with waterhaving a temperature not to exceed 40 0., and drying said article. I

5. The method of producing a transparent article from a metalsalt-alcohol solution of a synthetic linear polyamide which comprisesdissolving an interpolymer prepared from hexamethylene diammoniumadipate and hexamethylene diammonium sebacate in the proportion of 30%of the former to 70% of the latter in a calcium chloride-methanolsolution, forming an article from said solution, evaporatingsubstantially all the alcohol from said formed article at a ratesufliciently slow to prevent the formation of bubbles therein and beforesaid article comes into contact with moisture in greater concentrationthan an atmosphere having a relative humidity above 50%. and washing themetal salt from said article with water having a temperature not toexceed 40' 0., and drying said article.

6. The method of producing a transparent film from a metal salt-alcoholsolution of a synthetic linear polyamide'which comprises dissolving asynthetic linear polyamide having a quench index above mils in asolution taken from the group consisting of metal salt-methanol andmetal salt-ethanol solutions, forming a film from said solution,evaporating substantially all he alcohol from said film at a ratesufiiciently how to prevent the formation of bubbles therein and beforesaid film comes into contact with moisture in greater concentration thanan atmosphere having a relative humidity above 50%, and washing themetal salt from said film with water having a temperature not to exceed40 C., and drying said film while maintaining the same under suflicienttension to prevent the formation of wrinkles therein.

7. The method of producing a transparent film from a metal salt-alcoholsolution of a synthetic linear polyamide which comprises dissolving asynthetic linear polyamide having-a quench index above 125 mils in acalcium chloride-methanol solution, forming a film from said solution,evaporating substantially all the alcohol from said film at a ratesufficiently slow to prevent the formation of bubbles therein and beforesaid film comes into contact with moisture in greater con' centrationthan an atmosphere having a relative humidity above 50%, and washing themetal salt from said film with water having a temperature not to exceed40 C., and drying said film while maintaining the same under sufiicienttension to prevent the formation of wrinkle therein.

8. The method of producing a transparent film from a metal salt-alcoholsolution of a synthetic linear polyamide which comprises dissolving asynthetic linear interpolyamide having a quench index above 125 mils ina calcium chloride-methanol solution, forming a film from said solution,evaporating substantially all the alcohol from said film at a ratesufiiciently slow to prevent the formation of bubbles therein and beforesaid film come into contact with moisture in greater concentration thanan atmosphere having a relative humidity above 50%, and washing themetal salt from said film with water having a temperature not to exceed40 C., and drying saidfilm while maintaining the same under sufliclenttension to prevent the formation of wrinkles therein.

9. The method of producing a transparent film from a metal salt-alcoholsolution of a synthetic linear polyamide which comprises dissolving aninterpolymer prepared from hexamethylene di ammonium adipate andhexamethylen diammonium sebacat having a quench index above 125 mils ina calcium chloride-methanol solution, forming a film from said solution,evaporating substantially all the alcohol from said film at,

a rate sufiiciently slow to prevent the formation of bubbles therein andbefore said film comes into contact with moistur in greaterconcentration than an atmosphere having a relative humidity above 50%,and washing the metal salt from said film with water having atemperature not. to ex ceed 40 C., and drying said film whilemaintaining the same under sufficient tension to prevent the formationof wrinkles therein.

10. The method of producing a transparent film from a metal salt-alcoholsolution of a synthetic linear polyamide which comprises dissolving aninterpolymer prepared from hexamethylene diammonium adipate andhexamethylene diammonium sebacate in the proportion of of the former to70% of the latter in a calcium chloride-methanol solution, forming afilm from said solution, evaporating substantially all the alcohol fromsaid film at a rate sufiiciently slow to prevent the formation ofbubbles therein and before said film comes into contact with moisture ingreater concentration than an atmosride-ethanol solution, forming anarticle from said solution, evaporating substantially all the alcoholfrom said formed article at a rate sufficiently slow to prevent theformation of bubbles therein and before said article comes into contactwith moisture in greater concentration than an atmosphere having arelative humidity above 50%, and washing the metal salt from saidarticle with water having a temperature not to exceed 40 C., and dryingsaid article.

12. The method of producing a transparent article from a metalsalt-alcohol solution of a synthetic linear polyamide which comprisesdissolving a synthetic linear interpolyamide having a quench index above125 mils in a calcium chloride-ethanol solution, forming an article fromsaid solution, evaporating substantially all the alcohol from saidformed article at a rate sufliciently slow to prevent the formation ofbubbles therein and before said article comes into contact with moisturein greater concentration than an atmosphere having a relative humidityabove and washing the metal salt from said article with water having atemperature not to ex-. ceed 40 C., and drying said article.

13. The method of producing a transparent article from a metalsalt-alcohol solution of a synthetic linear polyamide which comprisesdissolving an interpolymer prepared from hexamethylene diammoniumadipate and hexamethylene diammonium sebacate having a quench indexabove 125 mils in a calcium chloride-ethanol solution, forming anarticle from said solution, evaporating substantially all the alcoholfrom said formed article at a rate sufiiclently slow to prevent theformation of bubbles therein and before said article comes into contactwith moisture in greater concentration than an atmosphere having arelative humidity above 50%, and washing the metal salt from saidarticle with water having a temperature not to exceed 40 -C., and dryingsaid article.

14. The method of producing a transparent article from a metalsalt-alcohol solution of a synthetic linear polyamide which comprisesdissolving an interpolymer prepared from hexamethylene diammoniumadipate and hexamethylene diammonium sebacate in the proportion of 30%of the former to 70% of the latter in a calcium chloride-ethanolsolution, forming an article from said solution, evaporatingsubstantially all the alcohol from said formed article at a ratesufliciently slow to prevent the formation of bubbles therein and beforesaid article comes into contact with moisture in greater concentrationthan an atmosphere having a relative humidity above 50%, and washing themetal salt from said article with water having a temperature not toexceed 40 C., and drying said article.

15. The method of producing a transparent film from a metal salt-alcoholsolution of a synthetic linear polyamide which comprises dissolving asynthetic linear polyamide having a quench index above 125 mils in acalcium chloride-ethanol solution, forming a film from said solution,evaporating substantially all the alcohol from said film at a ratesufficiently slow to prevent the formation of bubbles therein and beforesaid film comes into contact with moisture in greater concentration thanan atmosphere having a relative humidity above 50%, and Washing themetal salt from said film with water having a temperature not to exceed40 C., and drying said film while maintaining the same under sufficienttension to prevent the formation of wrinkles therein.

16. The method of producing a transparent film from a metal salt-alcoholsolution of a synthetic linear polyamide which comprises dissolving asynthetic linear interpolyamide having a quench index above 125 mils ina calcium chloride-ethanol solution, forming a film from said solution,evaporating substantially all the alcohol from said film at a ratesufiiciently slow to prevent the formation of bubbles therein and beforesaid film comes into contact with moisture in greater concentration thanan atmosphere having a relative humidity above 50%, and washing themetal salt from said film with water having a temperature not to exceed40 C., and drying said film while maintaining the same under sufiicienttension to prevent the formation of wrinkles therein.

1'7. The method of producing a transparent film from a metalsalt-alcohol solution of a synthetic linear polyamide which comprisesdissolving an interpolymer prepared from hexamethylene diammoniumadipate and hexamethylene diammonium sebacate having a quench indexabove 125 mils in a calcium chloride-ethanol solution, forming a filmfrom said solution, evaporating substantially all the alcohol from saidfilm at a rate sufficiently slow to prevent the formation of bubblestherein and before said'film comes into contact with moisture in greaterconcentration than an atmosphere having a relative humidity above 50%,and washing the metal salt from said film with water having atemperature not to exceed 40 C., and drying said film while maintainingthe same under suificient tension to prevent the formation of wrinklestherein.

18. The method of producing a transparent film from a metal salt-alcoholsolution of a synthetic linear polyamide which comprises dissolving aninterpolymer prepared from hexamethylene diammonium adipate andhexamethylene diammonium sebacate in the proportion of 30% of the formerto 015 the latter in a calcium chloride-ethanol solution, forming a filmfrom said solution, evaporating substantially all the alcohol from saidfilm at a rate sufliciently slow to prevent the formation of bubblestherein and before said film comes into contact with moisture in greaterconcentration than an atmosphere having a relative humidity above 50%,and washing the metal salt from said film with water having atemperature not to exceed 40 C., and drying said film while maintainingthe same under sufficient tension to prevent the formation of wrinklestherein.

ORRINGTON EMIBRY DWYER.

